Before cell division, the DNA carrying the genetic information has to duplicate by a DNA-polymerase. This requires among other necessary components the presence of large amounts of substrates, i.e. the presence of all four deoxyribonuclotides, e.g. deoxy Thymidine Tri-Phosphate (TTP, FIG. 2C). TTP is synthesized de novo according to the pathway illustrated in FIG. 1. In addition, mammals and many other species encode the TK, which exclusively is expressed in G1-S cell cycle stage and salvages Thymidine (T, FIG. 2A) into the metabolism by phosphorylating it to Thymidine Mono-Phosphate (TMP, FIG. 2B) (FIG. 1) (1, review). Determination of incorporated radioactive T into the DNA of cells in culture has thus traditionally been used as a measure of the rate of cell division. TK activity in human plasma or serum from healthy individuals and patients affected with various tumour diseases was shown during the 1980-ies (2, 3). Absence of TK activity in the spinal fluid was found in healthy individuals, while various levels were found in patients with brain tumours (4). Since then TK analysis has become more common in clinical medicine, especially in connection with different blood malignancies (5-10).
There exist two cellular genes encoding TK, TK1 being the major form in cell division is released into the serum as s-TK. This is the form over-expressed in serum in tumour disease and is measured using most of the current assays including the assay according to the invention. The TK2 form exists purely in the mitochondrion. Besides being localized to the mitochondrion TK2 also has a different substrate spectrum and kinetic parameters than TK1 (1).
The radioactivity included in the procedure in the hitherto only commercially available laborious serum TK (s-TK) assay has hampered its spread and use in clinical medicine (2). Being directly correlated to the degree of tumour cell division in vivo, s-TK is a complement to many tumour markers that merely indicate the tumour presence by over-expression of foetal antigens or products specifically produced by the differentiated cell. For example, prostate specific antigen (PSA) is an early marker of excess amounts of prostate cells, and the elevations of the level of antigen are detected long before the TK activity elevation. However, when the tumour starts to de-differentiate and starts to grow faster, PSA antigen (Kallikrein 3) disappears while increased s-TK activity is found (11). Thus, in addition to its use for monitoring tumour cell replication in patients, with or without therapy, the s-TK activity and alteration in its kinetics also singles out a patient with regard to a need for change in therapeutic treatment. The importance of a given s-TK level for the patient's outcome varies between different tumour types and relates naturally both to type of therapy installed, age of patient and other factors which make s-TK an important marker to include in multivariate models for use in clinical medicine (12).
As mentioned, in the prior art, the TK has either been detected by its activity or by procedures detecting the physical presence of the TK1 protein (24 kd gene expressed size) (13). The latter procedures pick up both active and inactive enzyme. Thus, these protein-detection assays correlate poorly to s-TK activity. This is not an unexpected phenomenon as TK1 is only active in the G1 to S stages of the cell cycle, i.e. at cell division, while the TK1 protein, or degradation products thereof, might have a different half-life. Thus, only the state of art measuring TK activity will be addressed here.
In general, all TK activity assays consist of an incubation of the sample to be analyzed with T or an analogue thereof, in presence of a phosphate donor (e.g. Adenosine Tri-Phosphate, ATP) and Mg2+ (or Mn2+). Thereafter the amount of product, i.e. TMP or any T-analogue monophosphate is determined as follows:                A) In the early described TK activity assays, 3H-labelled thymidine (T) was used as substrate. TMP was bound to Di-Ethyl Amino Ethyl (DEAE)-charged filter paper, which was subsequently washed in order to remove unused T substrate. After drying the filter paper the 3H radioactivity was measured in a beta-counter. However, still radioactive tests are developed which indicate the persistent interest in using s-TK as a marker for cellproliferation (14).        B) In the commercially available Prolifigen®TK-REA (DiaSorin S.p.A. Saluggia, Italy) assay, radioactive lodo-deoxy Uridine (125IdU) is used as substrate for TK. The Mono-Phosphorylated product; lodo-deoxy Uridine Mono-Phosphate (125IdUMP), is separated by binding it to self-sedimenting Aluminium Hydroxide (Al(OH)3) which is subsequently washed in order to remove unused 125IdU. The 125I radioactivity bound to the Al(OH)3 powder is subsequently counted in a gamma-counter (2, 3).        C) The recently published non-radioactive TK assays (15, 16) disclose a similar technology to quantify TK1 activity. In these assays either Azido-Thymidine (AZT, FIG. 2D) or Bromo-deoxy Uridine (BrdU, FIG. 2A) was used as substrate, and the respectively formed Monophosphate (AZTMP or BrdUMP FIG. 2B) was quantified by analyzing the capacity of the reaction solution to compete with the binding of enzyme-labelled AZTMP or BrdUMP to an anti-AZTMP or anti-BrdUMP antibody immobilized on the bottom of wells in a second 96 well microtiter plate. (i.e. detection by competitive ELISA).        
Albeit avoiding radioactivity, the hitherto known non-radioactive assay does not seem efficient enough for analyzing biological samples containing low levels of TK, since it measures the reduction of a bound tracer, which requires rather high amounts of TK product to be present in the analyzed sample (15, 16). This hampers good resolution of the TK activity in the normal range, which can differ more than three fold between young and old individuals, and particularly when the TK assay is used on biological samples devoid of measurable reference levels, such as spinal, pleural, ascites and other body fluids.
Recently a new recombinant host cell line was disclosed utilizing the complementing activity of TMPK, NdK alone or in combination with TK wherein the TMPK, NdK or the TK gene was deleted. That invention is directed to creating a recombinant cell line and kit including the cell line for screening of compounds affecting kinase pathways (17).